1. Field of the Invention
The present invention relates to the field of liquid crystal displaying, and in particular to a side-edge backlight module.
2. The Related Arts
Liquid crystal display (LCD) has a variety of advantages, such as thin device body, low power consumption, and being free of radiation, and is thus widely used. Most of the LCDs that are currently available in the market are backlighting LCDs, which comprise a liquid crystal panel and a backlight module. The operation principle of the liquid crystal panel is that liquid crystal molecules are interposed between two parallel glass substrates and the liquid crystal molecules are controlled to change direction by application of electricity in order to refract light emitting from the backlight module for generating images. Since the liquid crystal panel itself does not emit light, light must be provided by the backlight module in order to normally display images. Thus, the backlight module is one of the key components of an LCD. The backlight module can be classified in two types, namely side-edge backlight module and direct backlight module, according to the position where light gets incident. The direct backlight module arranges a light source, such as a cold cathode fluorescent lamp (CCFL) or a light-emitting diode (LED) at the back side of the liquid crystal panel to form a planar light source that directly provides lighting to the liquid crystal panel. The side-edge backlight module arranges a backlight source of LED light bar at an edge of a back panel to be located rearward of one side of the liquid crystal panel. The LED light bar emits light that enters a light guide plate (LGP) through a light incident face of the light guide plate and is projected out through a light exit face of the light guide plate, after being reflected and diffused, to thereby transmit through an optic film assembly and form a planar light source for the liquid crystal panel.
In a side-edge backlight module, the positioning of the light guide plate is of vital importance. Two requirements must be satisfied for the positioning of the light guide plate, one being precision of positioning and the second stability of positioning taking tolerance, thermal expansion, absorption of wet into consideration. The currently adopted solutions for fixing include the following; (1) using an arrangement of fixing hole (as shown in FIG. 1), wherein a plurality of fixing holes 102 is formed in a mounting side of a light guide plate 100 in order to be retained by positioning rivets 304 formed on a backplane 300; and (2) using a tetragonal L-shaped rubber block to effect positioning. These known solutions of positioning cannot properly handle expansion caused by being heated or absorption of wet. When a light guide plate expands, the dimensions thereon in both length and width directions increase. Taking a 55″ light guide plate made of PMMA (Poly(methyl methacrylate)) as an example, when the light guide plate is moved from room temperature (25° C.) into an oven of 40° C., the length increases by 2.8 mm A backplane made of an aluminum material will show an expansion length of 0.68 mm and the net amount of expansion will be 2 mm. By applying a simple mathematic model for calculation, if moving allowance for a light guide plate in the length direction in room temperature (25° C.) is zero, namely the light guide plate being incapable of moving in such a direction, then when the light guide plate undergoes thermal expansion by an amount of 2 mm, it can only bulge upward (as shown in FIG. 2). Calculation indicates that the bulging height H of the light guide plate is greater than 30 mm. Such a great amount of deformation would lead to the following problems. Firstly, the liquid crystal panel that is positioned on the top surface of the light guide plate will be broken or detached. The optic film assembly may undergo severe deformation and becomes waving (alternating of darkness and brightness for luminance). If the bulging is constrained through other measures, then the light guide plate would show multiple wavy warps that are of smaller bulging height, eventually leading to permanent deformation of the light guide plate, making it no longer possible to resume a flat and straight condition even after being cooled down. Thus, alternating of darkness and brightness of luminance may occur on the light guide plate.